Microwave radiation can be applied to a material in a number of ways, using single mode applicators, multimode applicators, traveling wave applicators, slow wave applicators, fringing field applicators and through free space. Each of the aforementioned methods of coupling microwave energy into a material has its advantages and disadvantages which usually depend on the dielectric properties, size and shape, of the materials to be processed and the type of processing (batch, continuous, liquid, plasma, etc) to be performed.
Efficient microwave energy transfer is a function of many variables as processing occurs. A number of these variables are material related, e.g., the material type and density and material temperature as well as the time history of both the material temperature and the applied electric field.
Other factors that influence coupling are related to the applicator, material geometry and size and the frequency or wavelength of the electromagnetic energy. Electromagnetic coupling depends on applicator size and geometry, material size and shape, the position of the material within the applicator, and even the relative sizes and shapes of the material and the applicator. In addition, both the applicator and material dimensions may change during heating which further complicates the efficient transfer of energy to the material.
Accordingly, a problem arises when attempting to generate a uniform microwave field across a relatively large surface for different material loads. As generally understood, if the volume of an applicator becomes too large, more than one electric field pattern can co-exist in the applicator, thereby making it multimode and introducing electric field non-uniformities. Current microwave applicators are incapable of generating a uniform microwave field across a surface that is relatively large compared to the wavelength of the radiation.
For instance, traveling wave applicators have some potential for providing uniformity. However, stray reflections, such as those that occur at the edges of a workpiece or any non-uniformity in the structure of the applicator can create standing waves leading to thermal non-uniformities. This is especially problematic in cases in which the material travels through more than one applicator and the dielectric properties of the material change depending on the processing conditions in the previous applicator.
An applicator design which shows some promise for applying uniform fields is a single mode applicator, provided that the fields can be extended over a sufficiently large region. This type of applicator can be tuned to specific electric field patterns (resonance modes) by varying the volume of the applicator.
One such approach is found in U.S. Pat. Nos. 4,507,588, 4,585,668, 4,630,566, 4,727,293, and 4,792,772 (Asmussen) all of which disclose methods and apparatuses in which a single mode resonant microwave applicator can be critically coupled by varying two separate, almost orthogonal variables, specifically the cavity length (by moving a short circuit) and the antenna position.
The Asmussen devices include a variable penetration antenna structure which acts to launch radiation into the applicator. The main advantage of the Asmussen device is that it enables complete critical coupling over a wide range of impedances (generated by the load in the applicator) and without the use of any external coupling structure. Critical coupling can thus be achieved by moving the short and the antenna appropriately.
By moving the flat part of the cavity wall (in a cylinder) in the z-direction (e.g., along the centerline of the cylinder), a wide range of electromagnetic modes can be established and maintained, even as the load varies (due to processing, e.g., temperature changing, material curing, etc.) However, one series of modes that can not be routinely excited are length independent modes, TM.sub.xy0. The resonant frequency of these modes are only dependent on the diameter of the loaded structure. As a result, if the load changes during processing (e.g., the dielectric properties change, due to increased temperature, curing, phase change in the material and so forth), the resonant frequency in the cavity changes from an initial, fixed processing frequency, usually 2450 MHz or 915 MHz (which are the ISM bands allowed by the Federal Communication Commission (FCC)). The Asmussen devices are thus not capable of maintaining certain modes in a controlled manner, namely the length independent modes (TM.sub.xy0) because these modes are dependent on the diameter of the applicator only.
In general, to process wide objects in a continuous manner, such as a web or sheet like product, as found in the paper industry, lumber industry (plywood) or electronics industry (in pre-impregnated cloth for circuit board manufacture), it is desirable to be able to (i) provide a uniform electric field over the entire product for uniform heating; (ii) vary the applicator to allow for variations in the dielectric properties of a continuously moving workpiece and, thus, vary the coupling of the radiation to the product; and (iii) control the microwave power reaching the product to control the temperature-time profile of the web.
The electric field pattern sustained by the TM.sub.0y0 series of modes, where y=1, 2 or greater, is oriented along the z-axis of the applicator and is of constant intensity along the entire length of the applicator for an empty cavity. This is an ideal mode for the processing of a web-like material. Referring to FIG. 1 (a mode chart), it can be seen that the TM.sub.010 mode is independent of the cavity length. Therefore, a low loss, infinitely long applicator is capable of sustaining the same electric field intensity throughout the length.
However, the electric filed is only truely uniform if the dielectric material uniformly fills the entire length of the applicator (ie, the material to be processed is as wide as the microwave applicator is long and has the same thickness along its width). If this condition is not met, and some means used to correct the situation, the electric field will cease to become uniform at the edges of the workpiece and potentially for up to a significant distance into the workpiece. This occurs for 2 reasons, first, the discontinuity creates a evanescent field, and second, the discontinuity changes the resonant condition of the cavity.
Evanescent fields occur at all discontinuities, however, through careful design and setup they can be minimized. The largest evanescent fields occur at sharp boundaries with large variations in dielectric constant and the electric field vector in the direction normal to the boundary. To minimize this field we avoid these conditions as much as possible. This includes avoid sharp boundaries and avoiding boundaries where the electric field is normal to the surface. In addition, dielectric structures can be added that help to minimize the evanescent fields by minimizing the boundary condition. Usually, these matching structures match the boundary condition of the origional structure in such a manner as to cancle the origional evanescent field.
Discontinuties also affect the resonant condition of the cavity. This occurs since the resonant frequency of the applicator in the regions where no workpiece (load) is present is different from that of the region where the workpiece is present. If the resonant frequencies of these regions is similar enough (by the workpiece providing a relatively small load on the applicator) relatively efficient coupling between the regions can be maintained and hence a relatively uniform electric field strength across the workpiece is maintained. If there is a sufficient difference in the resonant frequencies of the regions, the difference in resonant frequencies is greater than the bandwidth of the regions (which is due to losses and can be measured by a network, typically using the frequency width of the resonance at half power)and minimal power is coupled between the regions and hence the edge of the workpiece acts as an impedance mismatch, resulting in a non-uniformity in the heating of the workpiece.
Accordingly, it is an object of the present invention to provide a means of effectively enhancing the uniformity of the microwave energy over the full width of the workpiece by effectively minimizing the impedance mismatch at the adges of a workpiece and providing improved energy field distribution over a wide area, as compared to the prior art.
It is also an object of the invention to provide an elongated cylindrical microwave applicator that allows a uniform electric and magnetic field to be applied to a sheet of material being transported therethrough, in a continuous manner.
It is a further object of the invention to provide an elongated cylindrical microwave applicator that launches radiation at more than one input and provides uniform electric and magnetic fields along its length.